Process and composition for treating aluminum alloys



United States Patent Cfihce 3 ,003,896 Patented Oct. 10, 1961 3,003,896 PROCESS AND COMPOSITION FOR TREATING ALUMINUM ALLQYS Earl W. Kendall, San Diego, Calif., assignor to Rohr Aircraft Corporation, Clmla Vista, Calif., a corporation of California No Drawing. Filed Jan. 17,1958, Ser. No. 709,442 8 Claims; (Cl. 134-3) This invention relates to a process for'treating aluminum-rich alloys to provide an exposed surface thereon which is clean and substantially free from metallic oxides.

In the manufacture of many aircraft assemblies it is customary to electrically weld two or more sheet metal parts together which are made of alloys composed mainlyof aluminum. It has been found that thin films of oil', dirt and/or metallic oxide on the faying' surfaces of the parts obstruct the passage of the electric current and cause poor welds. It has also been found that the presence of foreign matter or oxides on the faying surfaces to' be welded together causes the electrical surface resistance to increase markedly and that to secure spot Welds of high strength, the surface or contact resistance between the parts should not exceed 100'microhms.

It is a main object of my invention to provide a methed for treating alloys having a high aluminum content which will remove all surface dirt therefrom and also remove any oxide film present on their surfaces;

A further object is to provide such a treatment which leaves the surface of commercial aluminum alloy sheet" so smooth and glossy that it has a pleasing appearance and reflects light well.

Another object is to provide a treatment of the type described which is effective on aluminum alloys of many different chemical compositions.

A still further object in a treatment of the type descrihedis to provide etching compositions which may be placed in aqueous" solutions forthe' purpose of treating alloys as aforedescribed.

Additional'objectswillbecome apparent as the description proceeds.

In treating sheets of aluminum-rich alloys which are to be spotwelded together, the sheets as they come from the mill are first immersed in or treated with the vapor of trichloroethylene, ethylene dichloride or other known organic solvent to remove any oil or grease film on their surface. The sheets are next cleaned with an inhibited alkaline cleaner and rinsed with mineral-free water. Next the sheets are immersed for from 3 to 5 minutes in an aqueous solution of the composition:

Percent by weight Nitric acid (HNO 15 to 25 Chromic acid (CrO 0.5 to 1.5 Water Balance the optimum content of nitric acid being approximately 22 percent. It is not necessary to heat this solution since it is effective within the temperature range of 70 to 90 F. The chromic acid acts as a passivating agent to somewhat inhibit the etching action of the nitric acid. This solution performs several functions in that it:

(1) Removes small particles of foreign metal which have become embedded in the surface of the sheet during the manufacturing process.

(2) Removes certain oxide and scale formations resulting from previous heat treatment or other exposure to elevated temperature.

(3) It brightens the surface of the sheet somewhat yet the specular gloss already present is not destroyed.

(4) It conditions the metal surface so that a more uniform action is secured in a deoxidizing medium described below.

The sheets are rinsed with mineral-free water anddeoxidized in a solution of the composition:

The proportions given for the citric acidand sermon: um bifluoride represent dry chemicals ranging frofritwo to thirty parts by weight of citric acid to one partby weight of ammonium bifiuoride. V g

Preferably 2 ounces of the citric acid and 0.1- ounceof ammonium bifluoride are used per gallon of water. Immersion in the solution for from 1 to 5' minutes at a temperature between 70 and 90 F. dis'solv'es the oxidesofi the surface of the sheets. The citric acid acts as a butter and also as a cleaning agent or des'mutter', the amm mum bifiuoride being the active etching a'g'enfi Ammonium bifluoride is to be taken as exemplary of soluble fl'u'o' as salts suitabie for the purpose. The citric acid, likewise; is to be taken as exemplary of soluble carboxylic acids suitable for the purpose. The citric acid, like other water soiuble carboxylic acids, serves as an organic reducing agent to prevent formation of oxides on the surface of the sheets. The citric acid, as a buffer serves to inhibit somewhat the'etching' action of the ammonium bifh'l'ori'd'e' whereby the immersion time may be several miiiutes as aforenoted. This solution thus dissolves and removes any oxide film of aluminum and other metals composing" the alloy.

The sheets are next thoroughly rinsed in mir'ieral-free water and then immersed in the aforedesc'ribed b'ath of nitric and chromic acids for from 2 to 3' minutes. This solution dissolves small dark particles of smut whicnmay have formed on the sheets while in the ammonium-Bi: anon-as etch and leaves the surface of the sheets clean and free of oxides. I

Thesheets are'now thoroughly rinsed-withmineral fr'e'e water and dried in an oven or by other known means at a temperature not over 110 F. It will be noted that this process requires the use of no heated baths, both the chemical solutions and all the rinse water preferably being at normal room temperature of about 70 F. but in no event higher than 90 F. The process leaves the surface of the sheets very smooth and with such a high gloss that it reflects incident light well. The electrical resistance at the surface of the sheets is less than 5 microhms. The process produces a sheet surface which is acceptable for electrical spot or seam welding, anodizing, alodizing and other chemical conversion treatments.

The process is effective with the following widely used aluminum alloys in all physical conditions and having the approximate compositions noted in percent by Weight:

It is not necessary to segregate sheets of the above compositions under my process but all the sheets may be processed simultaneously, thus saving the operators time in sorting out sheets of the same composition and processing each lot separately.

This invention may be embodied in other forms or carried out in other ways without departing from the spirit or essential characteristics thereof. The present embodiment of the invention is therefore to be considered as in all respects illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.

Having thus described my invention, what I claim as new and useful and desire to secure by Letters Patent is:

1. An aqueous solution adapted to dissolve and remove metallic oxides on the surface of aluminum-rich alloys immersed 'therein to reduce the surface electrical resistance thereof below microhms, said solution consisting essentially of:

Water gallons About 1 Citric acid "ounces" About 1 to 3 Ammonium bifluoride ..do About 0.1 to 0.5

2. A process for cleaning an article composed of an aluminum-rich alloy which comprises the steps of subjecting the article to the action of an aqueous solution of nitric and chrornic acids; rinsing the article with water; and subjecting the article to the action of an aqueous solution consisting essentially of water, citric acid and ammonium bifluoride.

3. A process for cleaning an article composed of an aluminum-rich alloy which comprises the steps of subjecting the article to the action of a solution consisting essentially of about 22 percent nitric acid, 0.5 percent chromic acid, 77.5 percent water by weight; rinsing the article with water; and subjecting the article to the action of a solution consisting essentially of water, citric acid and ammonium bifluoride in the proportion water about 1 gallon, citric acid 1 to 3 ounces, ammonium bifluoride 0.1 to 0.5 ounce.

4. A process for cleaning an article composed of an aluminum-rich alloy which comprises the steps of subjecting the article to the action of an aqueous solution of nitric and chrornic acids; rinsing the article with water; subjecting the article to the action of an aqueous solution of citric acid and ammonium bifluoride; rinsing the article with water; and again subjecting the article to the action of an aqueous solution of nitric and chrornic acids.

5. A composition for use in a cold aqueous solution for etching aluminum-rich alloys to reduce the surface electrical resistance below 5 microhms consisting of from two to thirty parts by weight of citric acid to one part by weight of an etching agent comprising only ammonium bifiuoride.

6. A process as in claim 2 in which the alloy is immersed in the citric acid-ammonium bifiuoride solution for from 1 to 5 minutes at a temperature of to F.

7. A process for cleaning an article composed of an aluminum-rich alloy which comprises the steps of subjecting the article to the action of a solution consisting essentially of about 22 percent nitric acid, 0.5 percent chromic acid, 77.5 percent water by weight for from 3 to 5 minutes at a temperature from 70 to 90 F., rinsing the article with water; and subjecting the article to the action of a solution consisting essentially of water, citric acid and ammonium bifluoride in the proportion water about 1 gallon, citric acid 1 to 3 ounces, ammonium bitluoride 0.1 to 0.5 ounce for from 1 to 5 minutes at a temperature from 70 to 90 F.

8. A process for cleaning an article composed of an aluminum-rich alloy which comprises the steps of subjecting the article to the action of an aqueous solution of nitric and chrornic acids for from 3 to 5 minutes at a temperature from 70 to 90 F., rinsing the article with water; subjecting the article to the action of an aqueous solution of citric acid and ammonium bifiuoride for from 1 to 5 minutes at a temperature from 70 to 90 F.; rinsing the article with water, and again subjecting the article to the action of said aqueous solution of nitric and chrornic acids for from 2 to 3 minutes at a temperature from 70 to 90 F.

References Cited in the file of this patent UNITED STATES PATENTS 1,554,483 Bailey Sept. 22, 1925 1,890,214 Churchill Dec. 6, 1932 2,172,171 Meyer et a1. Sept. 5, 1939 2,553,937 Patrie May 22, 1951 2,687,346 McDonald Aug. 24, 1954 2,710,792 McDonald June 14, 1955 

2. A PROCESS FOR CLEANING AN ARTICLE COMPOSED OF AN ALUMINUM-RICH ALLOY WHICH COMPRISES THE STEPS OF SUBJECTING THE ARTICLE TO THE ACTION OF AN AQUEOUS SOLUTION OF NITRIC AND CHROMIC ACIDS, RINSING THE ARTICLE WITH WATER, AND SUBJECTING THE ARTICLE TO THE ACTION OF AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF WATER, CITRIC ACID AND AMMONIUM BIFLUORIDE. 